Well, does Sciter support CSS Flexbox? It doesn't ...

Features which the project actually needs will be decisive.

Does the project need Canvas? Sciter is the better choice.

Does the project need CSS Flexbox? Cobalt will be a better fit.

The binary size may play a role if all else is equal ...

Sciter supports flex units but not flexbox. Flex units (and grid layouts) are in Sciter since 2007, ~10 years before browsers got them. Flexibility is the must for desktop UIs so it was in the engine from the very beginning.

Sciter's flex units + CSS flow property is a superset of flexbox and grid features. For example paddings, margins, border widths, left|right|top|bottom can be expressed in flex units.

Everything that can be defined by flexbox can be expressed by flex units / flow:

https://terrainformatica.com/w3/flex-layout/flex-vs-flexbox....

Sciter does not support CSS Flexbox. It supports a more or less equivalent, but incompatible feature.

That's a critical distinction when you want to run the same codebase as you have on the web.

CSS was designed for running on different UAs and there is special mechanism built in it. If you take a look into source of that demo page, you will see:

   /* browser */
   @supports (display:flex)  
   {
     section { display: flex; }
     section > * { flex: 1; margin:1em; } 
     ...  
   }

   /* sciter */
   @supports (flow:horizontal)
   {
     section { flow:horizontal; }
     section > * { width:1*; margin:1em; }
     ...
   }

You should not assume that all browsers support same set of features. And needless to say, that Sciter is not a universal browser. And so is Cobalt in that respect - it is a browser for particular site / application.

> there is special mechanism built in it

Yes, a return to the dark days of UAs having their own proprietary extensions / prefixes to do the same thing. Why bother when there's an actual industry standard?

> You should not assume that all browsers support same set of features.

You can certainly target only user agents which support CSS Flexbox. I would even say it's the advisable default unless you have some very special needs.